The disclosure relates generally to an arrangement and method for guiding expired respiratory gas flow in a breathing circuit through a housing assembly for removing an undesired expired gas component of the respiratory gas flow before conveying for an inspiration of a subject. The housing assembly comprises a first port and a second port, one of the ports being for receiving the gas flow and another of the ports being for discharging the gas flow. The disclosure also relates to a breathing circuit for ventilating lungs of a subject.
Anesthesia machines are optimized for delivering anesthesia to a patient using volatile anesthetic agent liquids. In such systems, the anesthetic agent is vaporized and mixed into the breathing gas stream in a controlled manner to provide a gas mixture for anesthetizing the patient for a surgical operation. The most common volatile anesthetic agents are halogenated hydrocarbon chains, such as halothane, enflurane, isoflurane, sevoflurane and desflurane. Additionally, nitrous oxide (N2O) can be counted in this group of volatile anesthetic agents, although the high vapor pressure of nitrous oxide causes nitrous oxide to vaporize spontaneously in the high pressure gas cylinder, wherefrom it can be directly mixed as gas with oxygen. The anesthetizing potency of nitrous oxide alone is seldom enough to anesthetize a patient and therefore another volatile agent is used to support that.
Since the volatile anesthetic agents are expensive, they are effective greenhouse gases and further harmful to the atmospheric ozone layer, anesthesia machines have been developed to minimize the consumption of the gases. To keep patients anesthetized, a defined brain partial pressure for the anesthetic agent is required. This partial pressure is maintained by keeping the anesthetic agent partial pressure in the lungs adequate. During a steady state, the lung and body partial pressures are equal, and no net exchange of the anesthetic agent occurs between the blood and the lungs. However, to provide oxygen and eliminate carbon dioxide, continuous lung ventilation is required.
Anesthesia machines are designed to provide oxygen to the patient and eliminate carbon dioxide (CO2), while preserving the anesthetic gases. To meet these goals a re-breathing circuit is used. In this patient exhaled gas is reintroduced for inhalation. Before re-inhalation carbon dioxide must be removed from the gas, which is done with a carbon dioxide absorber. Before inhalation, the gas is supplied with additional oxygen and anesthetic agents based upon the patient demand. In this arrangement, the additional gas flow added to the re-breathing circuit can be less than 0.5 L/min although the patient ventilation may be 5-10 L/min. Such ventilation of the lung is carried out using a ventilator pushing inhalation gas with overpressure to patient lungs and then allowing that to flow out passively from the pressurized lungs to the breathing circuit.
Ventilation carries the breathing circuit oxygen to lungs for uptake to be burned in body metabolism. The outcome of this is CO2 that blood circulation transports to lungs wherefrom it becomes carried out with exhalation gas. Before re-inhalation the gas is guided through absorber for CO2 removal. Effective CO2 removal requires close contact with the breathing gas and the removing substance. To provide large contact area, the removing substance is therefore a surface of porous structure of granules that fill the cartridge. The form of this granular structure is guided by the flow resistance, the limitation of which is one key design goals of the breathing circuit. In absorber optimized for minimal resistance the gas flow path through the stacked granules is short and the flow distributes to wide area. In such structure the gas flows slowly because of large surface area providing time for reaction between the gas and absorbent granules.
Absorber canisters have two gas connections: One inlet for the gas flow carrying carbon-dioxide and one outlet. Between inlet and outlet the canister has a gas pathway. The absorber granules form part of this pathway during which the carbon dioxide is removed from the gas.
The CO2 absorption is based on chemical reaction in the absorption cartridge. Typically the reaction is based on the use of alkaline chemicals often referred as soda lime (mainly including calcium hydroxide) that react with aqueous CO2. Typical end results of this exothermic reaction are calcium carbonate and water. The air exhaled by the patient includes approximately 5% of CO2. A fresh absorber is able to purify the breathing air from CO2 virtually completely. When the absorption capacity is getting exhausted there is a gradual increase of CO2 in the air downstream the absorber. A typical clinical practice is that latest when the inspiratory air reaches CO2 content of 0.5% the absorber unit needs to be replaced.
The CO2 absorption takes place in the soda lime bed inside the absorber cartridge. Depending on the absorber, such as container geometry, absorbent chemistry and grain characteristics, and the clinical factors, such as the amount of exhaled CO2, respiratory rate, etc, as well as the anesthesia machine set ups, such as used fresh gas flow, the absorbent volume and specifically the absorbent bed height required for complete CO2 removal change. This height or zone in the absorbent bed required for appropriate absorption of CO2 is often referred as “mass transfer zone”. Due to the characteristics of the absorption reaction and specifically the mass transfer zone a single absorber unit always includes a remarkable amount of unused absorbent at the time when it needs to be replaced to maintain below 0.5% CO2 levels since the required mass transfer zone at that moment is bigger than there is fresh absorbent remaining.
The problems related to the inability to fully use a single absorber do not exist in all anesthesia machine designs, in case there are two identical absorber units on top of each other. The absorbers are not intended to be replaced simultaneously but individually only after the individual absorber is fully exhausted. In practice this is accomplished by having the more exhausted absorber being exposed to the CO2 rich gas first. Even when it reaches the point where it has not enough capacity to absorb all the CO2 the second more fresh unit downstream is still capable of absorbing the remaining CO2. When the first absorber is fully exhausted it should be removed and the other partially used absorber can be mounted to the place where the first absorber used to be and a new fresh absorber is placed to the spot where the second absorber used to be. And the absorber changing cycle starts over again. On the other hand there are some concerns that the benefits of a twin absorber design may not be fully exploited if it is not easy or intuitive enough how to manage the swapping of the absorber units. Specifically, how to make secure the right absorber unit is moved from one port to another and that the right unit replaced with a new one.
However, there are some benefits with the more recently developed single absorber designs over the twin absorber assembly. One of them is the fact that in the single absorber designs available in the market place the gas return path is integrated into the absorber assembly and hence they require no additional conduit for return gas. In the well-known twin absorber design the gas return conduit is a part of the anesthesia machine—not integrated into the absorber. This means that even if a care giver uses disposable twin absorbers that are not serviced but simply disposed after use there still is the gas return conduit in the anesthesia machine that needs cleaning to avoid cross contamination. Also, all the parts in the breathing circuit add up the total air volume. However, a minimized air volume is preferable. The benefits include smaller total amount of anesthetic agents required as well as smaller system gas compliance.